Recent research has revealed that a proportion of the human population suffers some deficiency of stereopsis. Some of these individuals are stereoblind, unable to appreciate stereoscopic depth over the entire range of retinal disparity which normally results in stereopsis. Others are stereo-anomolous, selectively unable to perceive stereopsis over either the near or far depth disparity range. Most recent general theories of binocular combination attribute both stereopsis and sensory fusion to the same underlying neural mechanisms and suggest that stereo deficiencies may result from the absence of such mechanisms. Previous research by the principle investigator suggests that binocular summation may be an appropriate indicator of a capacity for sensory fusion. The aim of the proposed research is to examine binocular detection performence in stereoblind and stereo-anomolous observers to determine whether they exhibit summation when test stimuli are presented in a binocular configuration for which the observer is unable to perceive stereopsis.